GSM-850

Dieter Spaar spaar at mirider.augusta.de
Mon Aug 22 15:30:24 UTC 2011


Hello,

I don't know if anyone has already successfully used OsmocomBB
with GSM-850. It seems that although receiving on GSM-850 works,
there are issues when transmitting. The phone produces a signal
on the correct ARFCN, however the BTS can't make use of it.

The problem seems to be "I/Q-swap", something which can be turned
on in the DSP independently for receiving and transmitting. So far
it seems that this was not needed, at least for receiving and also
for tranmitting when using GSM-900/1800/1900. However for GSM-850
it seems to be essential for transmitting.

It is not clear if different phones behave differently, this might
be the case. Also it might be the case that some phones require
to turn on "I/Q-swap" when receiving on certain bands.

Considering all those uncertainties of "I/Q-swap" the appended patch
should be considered as a suggestion, it might be neccessary to make
it dependend on the phone and also to introduce a similar function
for receiving.

At least with the patch a C122 GSM-850/1900 phone now works on GSM-850
too.

Thank you very much to Harald who remembered that there was this
"I/Q-swap" thing for the DSP.

Best regards,
  Dieter

Dieter Spaar, Germany                           spaar at mirider.augusta.de
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